Nodal-ALK7介导的β细胞内源性调节对β细胞功能的影响及其机制

Diabetes is a leading cause of death by diseases worldwide. Excessive loss of islet β-cell mass mainly due to apoptosis is a major cause for the development of diabetic hyperglycemia. However, the underlying mechanisms are largely unknown. We have identified recently that the activin receptor-like kinase 7 (ALK7), a member of the serine/threonine kinase receptor of the transforming growth factor-β (TGF-β) superfamily, is expressed in the rodent islet β-cells. We have demonstrated that activation of ALK7, induced by high glucose, free fatty acids or pro-inflammatory cytokines, inhibited cell proliferation and enhanced apoptosis in the β-cells through a mechanism involving Akt kinase inactivation and activation of the Smad-caspase-3 signaling pathway(s). However, the mechanism(s) underlying the regulation of ALK7 in the β-cells is not identified. Our most recent findings showed that Nodal, another TGF-β superfamily member, is endogenously expressed in the β-cells, but not in the alpha cells or exocrine cells. We found that the apoptotic stimuli-induced apoptosis is associated with increased expression of Nodal in the β-cells. Notably, elevation of Nodal expression either by transfection or by treatment with Nodal protein significantly increased ALK7 expression in the β-cells, suggesting a critical role of Nodal-ALK7 pathway in modulating β-cell survival. Most previous studies only focused on the effects of exogenous factors in regulating the homeostasis of β-cell mass. Our recent studies suggested that the endogenous factors produced by the β-cell may play a critical role in modulating β-cell function and fate, which is exemplified by Nodal, the newly identified β-cell protein, acting as an autocrine regulator in the control of β-cell mass homeostasis. The objective of this study is to investigate 1) the molecular mechanism underlying Nodal production and secretion under normal and diabetic conditions; 2) the molecular mechanism underlying Nodal-ALK7 activation and its intracellular signaling mechanism; and 3) the regulatory mechanism of Nodal-ALK7 activation in the development of diabetes in mouse models. Identification and characterization of endogenous factor(s) that modulates β-cell function will not only provide new avenues to study islet biology, but also offer new insights into the biochemical nature of β-cell apoptosis, facilitating our efforts in the development of novel therapies for treating diabetes.

胰岛β细胞凋亡，β细胞群减少是糖尿病主要病因。已知高糖、高脂或促炎细胞因子是β细胞凋亡的诱发因素，确切机理尚未识别。ALK7是TGF-β家族一种受体蛋白。前期研究发现诱导ALK7可导致β细胞严重凋亡。最新研究显示多种凋亡诱发因子均能上调β细胞Nodal，引发AKL7介导的β细胞死亡。Nodal是TGF-β家族成员，胰岛内仅在β细胞特异表达，提示Nodal可能是ALK7配体蛋白。推测Nodal-ALK7调控失调是引起β细胞过度死亡，发生糖尿病的主要原因。本研究旨在用离体实验研究Nodal-ALK7的分子调控机制；用在体糖尿病小鼠模型解答Nodal信号通路失调是否引起β细胞群减少而发生糖尿病。本课题的完成，将阐明β细胞通过Nodal-ALK7及其信号链调控、维持β细胞群及血糖平衡的机理。探究识别β细胞内源性调节机制对β细胞群及功能的影响将为临床糖尿病防治提供新的预警因子和治疗靶点。

胰岛β 细胞凋亡引起的β 细胞群减少是引发糖尿病的主要原因。前期研究发现诱导ALK7 可导致胰岛β 细胞严重凋亡。最新研究显示多种凋亡诱发因子均能上调胰岛β细胞表达Nodal，引发AKL7介导的胰岛β 细胞死亡。我们推测 Nodal-ALK7调控失调是引起胰岛β细胞过度死亡，发生糖尿病的主要原因。本课题主要研究了胰岛β细胞 Nodal 自分泌的表达和调控机制，激活Nodal-ALK7信号通路对胰岛β细胞生长和凋亡通路的调节作用，自分泌因子nodal与insulin在胰岛β细胞增殖和凋亡过程中的拮抗作用，和自分泌因子γ-氨基丁酸（GABA）对胰岛β细胞增殖和凋亡的作用。我们的研究结果显示高糖，高脂促进nodal的表达上调，Nodal可以呈浓度依赖性的促进胰岛细胞凋亡，而insulin抑制高糖高脂引起的Nodal表达增加，以及ALK7-pSmad3 信号通路的激活；另一方面，Nodal抑制葡萄糖刺激的胰岛素分泌，机制是通过抑制Ca离子内流，并且Nodal拮抗insulin促进胰岛β细胞增殖的作用，机制是通过抑制AKT,ERK1/2,GSK-3b和β–catenin通路的激活；同时，GABA联合GLP-1受体（GLP-1R）激动剂Exendin-4，能够协同逆转细胞因子诱导的人胰岛β细胞凋亡，改善胰岛β细胞分泌胰岛素能力。我们的研究阐明了胰岛β细胞通过自分泌因子nodal，insulin，GABA调节细胞自身细胞凋亡的独特机制，即这些自分泌因子之间既有协同作用，也有拮抗作用。本课题的完成，拓展了人们对机体维持胰岛β 细胞群和血糖平衡机制的认识，为临床糖尿病防治提供了新的预警因子和治疗靶点，也为临床联合使用GABA和GLP-1R激动剂用于糖尿病治疗提供了新的理论依据。